The present invention relates to an improved planetary gear transmission, in particular for an electric vehicle, which provides two speed ratios with a simple configuration. The present invention also relates to an electric vehicle comprising such a planetary gear transmission.
Electric vehicles generally use at least one electric motor for propulsion. Output rotation and torque of the electric motor are transmitted to the wheels of the vehicle via a transmission.
For example, U.S. Pat. No. 6,513,401B2 discloses a transmission unit for axles of vehicles with electric drive, the transmission unit comprising a drive gear, a driven gear, and an intermediate idler gear meshing with both the drive gear and the driven gear. The speed ratio of the transmission unit can be varied by replacing the drive gear and the driven gear and displacing the axis of rotation of the idler gear. However, once the drive gear and the driven gear are set, the transmission unit provides only a fixed speed ratio.
It is known that an electric motor has a desired speed range in which the electric motor operates in a high efficiency region. The operation efficiency of the electric motor decreases when the speed of the electric motor falls out of this desired speed range. For an electric vehicle having a transmission with a single or fixed speed ratio, as that disclosed in U.S. Pat. No. 6,513,401B2, the electric motor has to operate at a low speed with a low efficiency in some conditions, especially when the running speed of the vehicle is low.
In order to increase the operation efficiency of the electric motor by operating it in the high efficiency region, a transmission which provides more than one speed ratio could be adopted. As an example, CN2875938Y discloses a two-speed automatic transmission for electric vehicles, the transmission comprising two gear pairs for providing different speed ratios, an output shaft which is able to be selectively coupled with either of the two gear pairs, and a splined hub fixed to the output shaft and configured to be displaced by means of a lever. When the vehicle runs at a low speed, the transmission is switched automatically to provide a higher speed ratio so that the electric motor operates at a higher speed, which results in higher efficiency of the electric motor.
However, a transmission with more than one speed ratio generally has a complex configuration. For example, two gear pairs are used in the automatic transmission disclosed in CN2875938Y. Such a transmission may be expensive or have a large size.
It is thus highly desirable to have a transmission which can provide more than one speed ratio with a simple configuration.